Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “Nodes”, “B node”, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
D2D Communication
Recent developments of the 3GPP Long Term Evolution (LTE) facilitate accessing local IP based services in a home, an office, a public hot spot or even outdoor environments. One of the important use cases for a local IP access and local connectivity involves the direct communication between devices such as user equipments in the close proximity of each other, typically less than a few 10 s of meters but sometimes up to a few hundred meters.
This direct mode or Device-to-Device (D2D) enables a number of potential gains over the traditional cellular technique where two devices communicates vi a cellular access point such as e.g. a base station because D2D devices are often much closer to one another than the cellular devices that have to communicate via the cellular access point.
One gain is capacity gain which may comprise reuse gain and hop gain. Regarding the reuse gain, D2D communication may provide reuse of radio resources such as e.g. Orthogonal Frequency Division Multiplex (OFDM) resource blocks, reuse gain. Regarding the hop gain, D2D communication provides a D2D link providing a single hop between the transmitter and receiver points as opposed to the 2-hop link via a cellular access point.
Another gain is peak rate gain, which may comprise proximity gain. D2D communication may further provide high peak rates due to the proximity and potentially favorable propagation.
A further gain is latency gain. When the UEs communicate over a D2D link, base station forwarding is short cut and the end-to-end latency can decrease.
The first step in the establishment of a D2D link is that the devices discover the presence of their peer. During the discovery process, one device is assumed to be in D2D slave role, and the other user equipment in D2D master role. To implement peer discovery, the D2D master device broadcasts signals, indicating its capability to provide certain service, and the D2D slave device tries to discover the D2D master device which may provide a required service. These signals that the master device broadcasts are referred to as beacon signals. Note that a single device may play both roles, i.e. master and slave in different occasions, or even simultaneously.
In order to broadcast the beacon signals, master devices use so called Peer Discovery Resources (PDR). In prior art systems, such as e.g. Bluetooth™, the PDR are made up channel of resources such as time and frequency channel resources. The notion of PDR is also used in other systems such as the FlashLinQ system provided by Qualcomm. FlashLinQ enables devices to discover each other automatically and continuously, and to communicate, peer-to-peer, at broadband speeds without the need for intermediary infrastructure. Generally, when multiple master devises broadcast beacon signals that use the same or overlapping PDR:s, it is referred to as a PDR collision. When a PDR collisions occur, the performance of the discovery procedure typically deteriorates. In order to avoid collisions existing systems may use, for example, frequency hopping used in Bluetooth or other distributed randomized schemes.
Embodiments herein generally relates to a D2D peer discovery scheme in a cellular based network such as e.g., LTE. Different from pure distributed network, in the case of infrastructure-assisted D2D, the cellular network may mediate in the discovery process by recognizing when two devices have a reasonable chance of successful D2D link establishment, and coordinating the time and frequency allocations for sending/scanning for beacons.
RSRP Measurement and Uplink TA Procedure in LTE
In existing systems, such as the 3GPP LTE system, there are standardized procedures and measurements that allow the radio access network such as the base stations, to manage handovers, set a user equipment transmit power and control link adaptation, and coding scheme selection by the user equipment. The following components of these procedures and measurements are referred to herein. LTE is used as an example, but the person skilled in the art will recognize that similar measurements and associated procedures are defined for other cellular systems, such as GSM, WCDMA, HSPA or WiMax.
The cellular base station periodically transmits i.e. broadcasts, DownLink (DL) Reference Signals (RS) with known transmit power levels and characteristics. For example, in LTE the RS sequence carries unambiguously the cell identity.
The user equipments in the cell served by that base station perform measurements on the received RS. For example, in LTE the user equipments perform a measurement referred to as Reference Signal Received Power (RSRP). The RSRP is defined as the linear average in Watts of the DL RS across a channel bandwidth of the cell.
Typically, the user equipments in the cell are configured by the base station to report some of the results of their measurements, including the RSRP measurements. In fact, the user equipment may report on the RSRP measurements from multiple cells, not only the cell that the user equipment is camping on, in order to help the base station to build an understanding of the radio position of the user equipment.
Another procedure of relevance in the current context is the uplink Timing Alignment (TA) procedure. The purpose of the TA procedure is to ensure that a user equipment's uplink transmissions arrive at the base station without overlapping with the UL transmissions from other user equipments. TA is controlled by the base station by configuring a timing advance quantity at the user equipment transmitter, which indicates to the user equipment the relative time relative to the received DL timing. In essence, timing advance is an offset, at the user equipment, between the start of a received downlink subframe and a transmitted uplink subframe. In LTE, the granularity of the TA parameter is 0.52 micro seconds and may be a maximum of 0.67 ms. When the user equipment is close to the base station, the TA may be set to 0, while the maximum value corresponds to about 100 km cell radius.
The updates of the TA to counteract the effects of user equipment mobility, changes in the propagation environment, etc, are performed by a closed loop mechanism between the user equipment and the base station. The closed loop mechanism means that the base station measures the received UL timing of user equipment data transmissions and issues TA update commands to instruct the user equipment to adjust its transmission timing accordingly.
As a summary, in existing systems, the user equipment and base station continuously performs measurements. The measurement results allow the base station to continuously have knowledge about the user equipments relative “radio distance” or “radio position” in the cell, e.g. in terms of path loss, multicell geometry and the received UL timing of UL transmissions and corresponding TA value calculated by existing methods.
In US 2009/0013081 A1 a D2D Peer Discovery (PD) scheme is disclosed, which is implemented in an autonomous way, Autonomous Peer Discovery (APD). That is, without any control from a central point such as e.g., an access point or base station. A challenge in this scenario is how to enable two peer devices nearby each other to meet in time and frequency. Without any co-ordination, this is only possible via a randomized procedure such that the peers sending and scanning for beacons at different times and/or frequencies, after a certain amount of time discover each other with high probability. Two drawbacks of this scheme are as follows.
It requires lots of PD Resources (PDR) available for a master device, within which some are chosen for beacon broadcasting randomly. To avoid beacon transmission collision from multiple master devices, there should be a large number of PDR available, so it is not resource efficient.
To catch the possible beacon transmission in these many resources, the slave devises have to monitor all PDRs, which is not power efficient.
A possible solution to increase the resource and power efficiency may be to assign PDRs in a scheduled way, i.e., network-assisted PD, e.g., Location information based PD (LPD). A master device informs the network which services it offers. This information changes rather infrequently and the corresponding update rates are low. In addition, the device also provides its position to the network. The network transmits the location information of master devices together with a service identifier (e.g. expression) to slave devices, either via broadcast and/or unicast. If unicast is used, a slave device can be updated with location information of nearby master devices only. In this case even slave devices need to provide the network with their position to enable the network to filter out far away master devices. A slave device may even inform the network which services it is interested in to further reduce potential master devices
Since this method requires a slave device to keep monitoring location information using, for example, Global Positioning System (GPS), it would cause extra user equipment power consumption due to GPS signal reception. Other positioning technologies involve the continuous reception of technology specific signals implying a similar problem.
Further, since this method requires all user equipments, masters and slaves to report the location information to the base station with certain cycle, and the base station would send these information to all interested neighboring slave devices, it would cause large signalling overhead, especially when performed via user equipment specific signaling.